Prosthesis components for replacing anatomical joints are well known in the art, including total hip replacement systems. These systems include acetabular components and femoral components which interact with the acetabular components to replicate the articulation between the head of a femur and an acetabulum, or cotyloid cavity, of a pelvic girdle. The acetabular component typically comprises two parts, a metal acetabular shell and a polyethylene liner for insertion into the acetabular shell.
Acetabular shells typically have openings in the shell, including screw holes, which give rise to at least two problems. First, wear debris that is generated from the articular movement between a femoral head component and the liner can migrate between the liner and the acetabular shell, pass through the openings in the acetabular shell, and cause damage to the bone into which the acetabular shell is implanted. Second, debris can also be generated as a result of minor movement between the liner and the acetabular shell, commonly known as micromotion. This debris can also pass through the openings in the acetabular shell and cause damage to the bone.
Some efforts have been made to address this second problem, including providing an acetabular shell with a polished inner shell surface so as to minimize the amount of debris generated. Additionally, several manufacturers provide shells and liners with a variety of tongue and groove like configurations, such as generally rectangular tabs and indentations for mutual engagement. While these interlocking means serve to reduce the amount of debris generated by micromotion, they fail to solve the problem of migration of debris from the prosthetic articular surface between the acetabular shell and liner. Thus, despite these efforts, large numbers of polyethylene particles still have access to the acetabular bone.
Another approach, which attempts to address both of the debris problems mentioned above, particularly for acetabular shells having screw holes, is to utilize what are known as “man-hole covers,” or plugs, for insertion into unused screw holes to prevent the migration of debris therethrough. The main disadvantage of this approach is that the plugs fail to prevent the migration of debris through holes that have bone screws inserted therethrough. The debris will still migrate around the screws and through the holes, and ultimately damage the bone. Additionally, many acetabular components are now made without screw holes to prevent migration of polyethylene debris into bone behind the metal shell, but this can compromise fixation of the shell to bone in cases in which screws are necessary.
The inventor herein is aware of only one design that obviates both of the problems mentioned above, but the usefulness of this design has its own limitations. The acetabular shell is provided with a tapered inner surface for interfacing with a liner having a corresponding taper. Because the taper interface must fit tightly with each other to be effective, it is virtually impossible to seat the polyethylene liner onto the inner dome surface of the metal shell. Because the liner only contacts the acetabular shell about the peripheral edge of the shell to create an annulus of contact, part of the liner, which extends into the acetabular shell, is unsupported. Although such a configuration does provide a sealing effect between the liner and the acetabular shell, the design is unsuitable as liners formed from polyethylene deform over time which interferes with the desired smooth motion between the head and liner. Moreover, because all the load exerted on the liner is concentrated at the peripheral interface with the acetabular shell, the thickness of the shell must be relatively large, which, for acetabular components of smaller joints, requires a relatively and unacceptably thin liner. If the tapered portions of the shell and liner are made large enough to avoid distortion of the polyethylene shell, then this leaves no room for screws to be placed through the most effective portion of the shell's dome.
What is needed is an acetabular component that provides a seal for the interface of the acetabular shell with the liner, that can substantially inhibit micromotion between the acetabular shell and liner, and that provides adequate support for the liner across substantially all of liner so that the liner can be formed from polyethylene.